1. Field of Invention
This invention relates to a semiconductor device, especially to a semiconductor device used as a power device.
2. Description of Related Art
A low temperature poly silicon thin film transistor (referred to as LTPS-TFT hereinafter) has been known as a TFT used in a pixel portion or peripheral circuits of a display device such as an LCD display device and an EL display device. The LTPS-TFT is a TFT having a poly-silicon that can be crystallized as an active layer at temperature low enough for a glass substrate. This TFT also has a high carrier-mobility. It is therefore suitable for a display device designed for driving and switching pixels formed a glass substrate.
Next, the configuration of the conventional LTPL-TFT will be explained by referring to FIG. 3, which is a cross-sectional view of the conventional LTPL-TFT. This figure only shows the configuration of the n-channel type TFT.
An insulating film 21 is formed on a glass substrate 20. An active layer 22 configured from low temperature poly silicon (poly-Si) is formed on the insulating film 21. A channel region 22c is formed between a source region 22s a drain region 22d of the active layer 22. The source region 22s and the drain region 22d each have an n+ layer with a high impurity concentration and an n− layer with a low impurity concentration, forming a LLD (lightly doped drain) configuration.
A gate electrode 24 is formed above the channel region 22c with a gate insulating film 23, which covers the active layer 22, between them.
An interlayer insulating film 25 is formed on the gate electrode 24 and the insulating film 23. A contact hole C2s is formed piercing the gate insulating film 23 and the interlayer insulating film 25 at the location corresponding to the source region 22s, and a source electrode 26s fills hole. Also, a contact hole C2d is formed piercing the gate insulating film 23 and the interlayer insulating film 25 at the location corresponding to the drain region 22d, and a drain electrode 26d fills the hole.
A passivation layer 27 that flattens and protects the surface of the TFT is formed on the area above the interlayer insulating film 25 including the areas above the source electrode 26s and the drain electrode 26d. The related technology is disclosed, for example, in Japanese Patent Application Publication No. 11-111998.
However, Joule heat is generated at the channel region 22c in the high resistance area during the electric current driving in the conventional LTPS-TFT when a large amount of electric current is applied, that is, when the TFT is used as a power device. The TFT is heated up from Joule heat, lowering the Ids and changing its threshold value and leading to the deterioration of the TFT quality.
Therefore, this invention is directed to a LTPS-TFT that can accommodate a large amount of electric current and prevent the quality deterioration from the heat during the application of the electric current driving.